Various methods of geophysical exploration have been developed to aid in the determining the nature of subterranean formations for exploratory oil and gas drilling. The seismic survey is one form of geophysical survey that aims at measuring the earth's geophysical properties. It is based on the theory of elasticity and therefore tries to deduce elastic properties of materials by measuring their response to elastic disturbances called seismic (or elastic) waves.
There are three major types of seismic surveys: refraction, reflection, and surface-wave, depending on the specific type of waves being utilized. Each type of seismic survey utilizes a specific type of wave (for example, reflected waves for reflection survey) and its specific arrival pattern on a multichannel record.
In reflection-type seismic surveys, the reflected seismic waves are detected at or near the surface by a group of spaced apart receivers called geophones, accelerometers, seismometers or similar transducers. These transducers are collectively referred to as “geophones” herein following industry convention, but it is understood that they could be any sensor that detects seismic energy. The reflected seismic waves detected by the geophones are analyzed and processed to generate seismic data representative of the nature and composition of the subterranean formation at various depths, including the nature and extent of hydrocarbon deposits. In this way, the seismic information collected by geophones can be processed to form images of the subsurface.
It has become common in many cases for the source of propagating elastic waves to be a hydraulically-operated, truck mounted vibratory source, more simply referred to as a “vibrator” in the art. There are other forms of energy sources for vibrators like electromechanical or pure electric. All of these systems typically generate vibrations or shock waves by using a reaction mass member that is actuated by a hydraulic or electric system and electrically controlled by a servo valve driven by a pilot sweep.
In a typical embodiment, a vibrator comprises a double ended piston rigidly affixed to a coaxial piston rod. The piston is located in reciprocating relationship in a cylinder formed within a heavy reaction mass. Means are included for alternately introducing hydraulic fluid under high pressure to opposite ends of the cylinder or for an electric coil and magnet type assembly to impart a reciprocating motion to the piston relative to the reaction mass. The piston rod extending from the reaction mass is rigidly coupled to a baseplate, which is maintained in contact with ground surface. Since the inertia of the reaction mass tends to resist displacement of the reaction mass relative to the earth, the motion of the piston is coupled through the piston rod and baseplate to impart vibratory seismic energy in the earth.
Typically, vibrators are transported by carrier vehicle or truck, and it is also known to prevent decoupling of the baseplate from the ground by applying a portion of the carrier vehicle's weight to the baseplate during operation. The weight of the carrier vehicle is frequently applied to the baseplate through one or more spring and stilt members, each having a large compliance, with the result that a static bias force is imposed on the baseplate, while the dynamic forces of the baseplate are decoupled from the carrier vehicle itself. In this way, the force from the vibrating mass is transferred through the baseplate into the earth at a desired vibration frequency. The hydraulic system forces the reaction mass to reciprocate vertically, at the desired vibration frequency, through a short vertical stroke.
A significant problem with conventional systems employing a vibrating baseplate to impart seismic waves into the earth is that the actual motion of the baseplate, and thus the actual seismic energy imparted to the earth, deviates from the ideal motion represented by the pilot signal. This difference can be caused by a variety of factors, including (1) harmonic distortion or “ringing” of the baseplate, (2) decoupling of the baseplate from the earth's surface commonly referred to as bouncing or “pogo-sticking,” (3) flexure of the baseplate, and (4) uneven ground resulting in inconsistent baseplate contact. Baseplate flexure is not only problematic from the standpoint of generating a distorted seismic signal, but it is also problematic because flexure of the baseplate contributes to structural failure of the baseplate through e.g., metal fatigue.
The differences between the pilot signal and the actual baseplate motion are problematic because, in the past, the pilot signal was used to pulse-compress the reflected seismic signal either through correlation (industry standard conventional “vibroseis”) or inversion based techniques like ZenSeis® or HFVS. Thus, where the actual motion of the baseplate differs from the ideal motion corresponding to the pilot signal, the pulse-compressed reflected seismic signal that is produced by correlation or more modernly by inversion will be inaccurate.
These problems are known to the industry and several of the vibrator manufactures have tried to address them through stiffer baseplates, different airbags and hydraulic modifications with varying degrees of success. To date though, there has not been an all-encompassing solution to the problem as fixes to one part of the problem tend to exacerbate another. Hydraulic vibrators are really quite well optimized and a credit to the manufacturers, but they still have issues that need to be solved.
Some attempts have been made to address this flexure issue. In EP2365357, also by Applicants, an improved baseplate having reduced surface area and reinforced walls is proposed. However, some degree of flexure is still inevitable with this design. Therefore, there is a need in the art for an improved baseplate design to correct the flexure problem and also prolong the life of the baseplate.